60 VERTEBRATE RESPIRATION 



enormous (Table 1). The lung forms a compact structure, how- 

 ever, as it is supported by the presence of connective tissue which 

 includes many elastic fibres. The elastic tissue is relatively 

 abundant in the finer branches of the bronchial tree but in the 

 larger branches there are quite distinct muscle layers. The cilia 

 on the epithelium permit a constant renewal of the fluid layer 

 through which diffusion must occur. They beat continuously 

 towards the mouth and remove foreign material from the 

 respiratory tract. 



(e) DIVING IN TETRAPODS 



The ability to spend a greater or lesser amount of time sub- 

 merged is found in all groups of living tetrapods and is known 

 to have occurred in many now extinct groups. In nearly all cases 

 oxygen continues to be obtained from the air although some 

 neotonic Amphibia, which remain tadpole-like when adult, 

 absorb it from the water by means of their external gills. The 

 problems facing poikilothermic divers are not so great as those 

 for homoiotherms because even on land they frequently pass 

 long periods without breathing. In diving birds and mammals, 

 however, the regular respiratory rhythm must be modified if 

 diving is to be longer than the normal period of respiration. Few 

 diving birds are able to submerge for longer than two minutes, 

 although ducks have been kept with their heads beneath the 

 water for at least fifteen minutes and survived when brought up 

 again. Diving times of many mammals are well authenticated 

 and some are given in Table 9. The longest dives are made by 

 certain whales but seals frequently submerge for fifteen minutes 

 or longer. Nevertheless, their average duration may be sub- 

 stantially less than the figures given in this table. 



In all cases the diving animal takes down a certain volume of 

 oxygen either in its lung or physico-chemically combined in the 

 blood, muscles, or tissue fluids. The extent of such stores has 

 been calculated for several divers and is given below for a seal, 

 whale, guillemot, and alhgator (Table 10). It is noticeable that 

 the site of the major oxygen supply varies for each animal. The 

 seal takes most oxygen down combined in its blood which has a 



